A man and woman, between 20 and 30 years old, were shot in the leg, Denver police tweeted. Their injuries were considered non life-threatening.

A third victim, a juvenile, was grazed by a bullet and walked into a nearby hospital, police tweeted.

Several other people suffered some minor injuries from running or being trampled, according to witnesses who talked to 7NEWS.

Police asked those attending the rally for possible photo or video of the shootings, and had no immediate motive for the shooting.

“Everybody in this world has a camera nowadays, so if anybody has any pictures that might assist us, we’d like those as well," said Denver police spokesman Sonny Jackson.

AIRTRACKER7 was over the area at the time of the shooting and streamed video of the crowd scattering and one man on the ground, surrounded by police officers. Several police motorcycles were parked nearby and paramedics arrived a few moments later to transport the man to Denver Health.

7NEWS reporter Russell Haythorn said he heard "four to five shots fired" at about 5:02 p.m. and the ambulances arrived while he was reporting live in the 5 p.m. newscast.

Kyle, a witness who was next to the outdoor Greek theater when the shots were fired, said, "lots of people had different ideas of what was happening. Some people were yelling 'Explosion!' some people were yelling, 'Gunshots!' We couldn't really find if it was coming from the stage or from the side because there were just so many people literally just running at people."

"We were standing in the center over there, they just started shooting," a witness in Broncos clothing told Haythorn.

"Did you see anything?" Haythorn asked.

"I didn't see it, but I could hear 'boom, boom, boom,' and I could see everybody just trampling and running," the witness responded.

Another witness said, "We heard somebody yell, 'Gun!' then a wave of people came."

"A gush of people just came running. We were like, 'Whoa.' People were dropping stuff," another witness added.

"I saw him fall, grabbing his leg," said Travis Craig, 28, who was at the celebration and witnessed the aftermath of the shooting. He said he used a belt to apply a tourniquet to the man's left leg.

"He was just screaming that he was in pain, and wanted to know where his girlfriend was. She was OK. And then the cops showed up real quick, like, less than a minute. They put him on ambulance and left."

A large number of police officers were already at the downtown Denver park watching the marijuana rally, the first 4/20 rally since the state legalized pot.

Denver police had said its officers were there to monitor crowd security in light of the attack at the Boston Marathon and not there to arrest anyone for public pot smoking, which is still illegal in Colorado.

Police now say they are looking for two men. They described the first shooter as a light complexioned black man, about 6 feet tall and weighing about 180 pounds. He was wearing a gray hoodie, black pants and a Carolina blue baseball cap.

The second person was described as a black man in a checkered black and white shirt. Jackson didn't have a description of that suspect's height and weight.

Denver police closed multiple roads including Colfax at Bannock, 13th at Bannock, 14th at Bannock and Colfax at Delaware as part of the investigation.

The shooting meant the cancellation of all 4/20 pot-related events at Civic Center Park for the weekend and the permit for the rally organizers has been revoked. The decision was mutual between both parties, park and recreation officials and the organizers of the rally, the city said.

“Our top priority is public safety,” said Lauri Dannemiller, manager of Denver Parks and Recreation. “We’ve met with the event organizers and we’re all in agreement that out of respect for the individuals that were injured and to allow Denver Police the ability to conduct a thorough investigation, events should be canceled for the remainder of the weekend.”

Miguel Lopez, who organizes the 4/20 rally, told 7NEWS in the 29 years the rally has been held in Denver this is the first time there's been a shooting.

The gathering was expected to draw 80,000 but it did not seem as packed as it had in years past, when it drew 50,000 people. Another marijuana-related event, the High Times Cannibis Cup, was also held in Denver on Saturday, the first time the event has been held outside of Amsterdam.

The rally drew many some people from out of the state, here to enjoy recreational marijuana legally.

Rapper Lil' Flip was performing when the shootings occurred.

Before the shooting, reggae music filled the air, and so did the smell of marijuana, as celebrants gathered by the park that sits in the shadow of the state Capitol.

Group smoke-outs were planned Saturday from New York to San Francisco. The origins of the number "420" as a code for pot are murky, but the drug's users have for decades marked the date 4/20 as a day to use pot together.

A citizen advocacy group that opposes marijuana proliferation, Smart Colorado, warned in a statement that public 4/20 celebrations "send a clear message to the rest of the nation and the world about what Colorado looks like."

"Does the behavior of the participants in these events reflect well on our state?" asked the head of Smart Colorado, Henny Lasley.

A few minutes after the shooting, the helicopter had to leave the park because of a temporary flight restriction related to the Rockies game.

Copyright 2013 Scripps Media, Inc. The Associated Press contributed to this report. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.